The thing about pushing is that it isn't JUST about getting help from gravity, because the uterus is an incredibly powerful muscle and it doesn't often 'need' gravity's help (though supine, against gravity, is definitely not good). Yes, I'm a fan of upright positions in many cases, but not in all. For a primip with a tight perineum, it can actually be better if she is sidelying or hands and knees, or even a mild knee chest slant--no need to allow gravity to speed descent if you want descent nice and slow for the sake of the perineum's ability to stretch enough to prevent tearing.
Also, apart from the power of the uterus and gravity's effects, there is the question of 'other' muscular resistance on the part of a mom. sometimes, sidelying can help her relax entirely, between and even with pushing contrax...remember, we are not forcing toothpaste through a tube, we're hoping for a mom to open and 'give' birth...let her body give way to baby's progress.
If I have any reason to think Shoulder Dystocia is possible for a mom/baby, then I also prefer not to add gravity to the mix. Again, you don't want too much force bringing baby's shoulder too tightly against that pubic bone--and the more relaxed mom can be, the less likely impaction will happen, or at least become real bad.
Sometimes, sidelying is best in helping baby find the way through the maze, as it were. And can produce the very strongest of all contrax--this varies by individual, of course. But there is no one position or type of position that works best for all, in terms of comfort, stretching perin., baby dancing descent, and/or power of contrax.
Other than telling women in advance as well as when 2nd stage starts that they probably ought to avoid supine/reclined positions that put weight on the tail/sacrum, I mainly invite women to find what feels best. Will suggest specific positions if needed, but usually women find what works/feels best for themselves without my 'coaching'.